It is critical to the future of nuclear power to solve the problem of long-term storage of high-level nuclear spent fuel. In connection with this, the nation's first underground waste repository site is proposed to be constructed in a formation of unsaturated welded tuff (a very hard igneous or volcanic rock) at Yucca Mountain, Nevada. The concept is the subject of public and scientific criticism, partly because local future climate and seismic events are uncertain. In addition, there are risk factors associated with the breakdown of the natural geological and the associated man-made barrier systems. Other concerns are possible future complications attending retrieval of the waste from the site for reprocessing, or alternative storage emplacement.
Two different waste emplacement layout approaches have been considered by the U.S. Department of Energy (DOE) in order to distribute heat generating waste evenly over an appropriate underground area in Yucca Mountain. The long, horizontal emplacement approach assumes long horizontally driven holes which are 100 to 200 meters in length, in which 14 to 18 canisters could be emplaced in series. Canisters are 0.7 meters in diameter and five meters high. This layout has the advantage of low drift wall temperatures, since heat is transferred directly into the rock heat sink area, farther from the surface of a drift. However, this approach presents emplacement and retrieval problems, and in response to these concerns, the long, horizontal emplacement concept has been rejected.
Currently, only the short vertical or horizontal hole approach is being considered. While short holes provide for easy retrieval, they also have disadvantages, namely, (1), the heat source is close to the surface of the emplacement drift or underground tunnel and extensive ventilating air is needed to cool down the drift wall for regular maintenance or retrieval, and (2) a large amount of underground excavation is needed to spread the heat load evenly over the waste site area, resulting in excessive construction and operating costs and an increased risk of failure in the engineered or geological barrier systems. Waste storage using tubular waste containers and an open air circulation system such as this is described in the U.S. Pat. No. 4,713,199, where the containers are emplaced into vertical holes of a concrete block incorporating channels and cooling is provided by air flowing between the containers and the channel wall.
Other suggested layouts and arrangements, however, can provide for easy retrieval. The following possibilities have been suggested by the Nuclear Regulatory Commission: single-package instead of multi-package emplacement, short holes and floor trenches, or alcoves in the drift wall. While these arrangements are advantageous for low-level nuclear waste, they are not suitable for high-level waste storage, because of the high heat and radioactive radiation load. One approach to the problem is described in U.S. Pat. No. 4,834,916, again using convective air cooling, with all its attendant problems, and where waste is stored in tubes placed within a room having at least one cold air inlet and a hot air outlet.
The methods described in the foregoing employ so-called open-loop ventilation circuits to control the temperature of the waste container and the storage area. Open-loop ventilation is potentially hazardous when applied to high-level waste, since contamination can be released from a leaking container. In addition, such systems also usually involve the added energy costs of providing a source of cooled air.
There are other methods for the removal of decay heat. A heat sink in the form of a heat conducting rod extending substantially along the length of a spent fuel rod is described in U.S. Pat. No. 4,326,918. Here the decay heat is conducted away from a sealed storage assembly for spent nuclear fuel.
A sealed storage complex is described in the U.S. Pat. No. 4,725,164, where the amount of excavation is minimized for safety purposes. An elastoplastic filling material is used which conducts the heat of the waste to the site area. Another two-phase cooling circuit is described in U.S. Pat. No. 3,706,630. In accordance with this patent, the decay heat is removed by water evaporation from the fluidic form of waste during disposal. The vapor travels through a closed pipeline to a surface-based cooler to condense. The condensate is re-used, after mixing with the waste and pumped back to the site. This solution represents a two-phase cooling system with a cooler on the surface, which is potentially hazardous, requires utilities, operating personnel and maintenance.